Physiological Principles of Estrous Synchronization
by Troy Smith for Angus Productions Inc.
NASHVILLE, TENN. (Aug. 5, 2010) — Estrous synchronization and artificial insemination (AI) have been called the most powerful and applicable technologies for genetic improvement of beef herds. And while synchronization products and protocols have changed over time, the basic physiological principles underlying how these products work have not changed. So said University of Missouri animal scientist Michael Smith during a presentation to the Applied Reproductive Strategies in Beef Cattle (ARSBC) Conference in Nashville, Tenn.
“Understanding the basic principles of the bovine estrous cycle and how estrous synchronization products affect the cycle is essential when choosing the best protocol for heifers or cows, and for determining what went wrong when pregnancy rates following a synchronized estrus are less than expected,” Smith said.
Reviewing the fundamentals of a normal 17- to 24-day cycle, Smith said the duration of estrus is generally 10 to 18 hours, though variation exists among individual animals. He explained how the estrous cycle is divided into three stages (follicular phase, estrus and luteal phase) and is regulated by hormones secreted by the hypothalamus, anterior pituitary gland, ovary and uterus. The hormones serve as chemical messengers that travel in the blood to specific target tissues. According to Smith, the development of estrous synchronization protocols for cycling females has involved three approaches to strategic hormone treatment.
The first approach requires long-term treatment (14 days) with progestin to inhibit estrous until the drug is withdrawn. Commercially available progestin products include melengestrol acetate (MGA), which is administered orally, and the CIDR® (controlled internal drug release) vaginal insert. According to Smith, progestins also are effective for initiating estrous cycling status in anestrous cows and prepuberal heifers.
The second approach to estrous synchronization involves injection of prostaglandin to induce corpus luteum (CL) regression. Smith warned that this treatment is only effective in already cycling cows and heifers. However, animals that are in the first five to six days of their cycles will not respond to treatment. Prostaglandin will not induce cycling activity.
Smith said most currently used protocols represent a third approach that combines the use of a progestin and prostaglandin. Such protocols are most effective in herds containing both cycling and non-cycling females. The ability to synchronize bovine follicular waves and initiate ovulation by incorporating injections of gonadotropin-releasing hormone (GnRH) has also allowed for fixed-time AI.
According to Smith, the success of estrous synchronization also hinges on selection of heifers and cows that have a high likelihood of responding favorably to a chosen protocol.
“With heifers, consider what the pregnancy rate of heifers from your herd has been during the last few years. Have they received growth promoting implants? Implants administered within 30 days of birth may impair normal development of reproductive organs. Consider whether your target breeding weight is realistic. And choose heifers with reproductive tract scores of 4 or better,” Smith advised.
When selecting cows as candidates for synchronized AI, Smith advised producers to again consider past pregnancy rates. He recommended cows be in body condition score 5 or greater. Also, synchronization should not be initiated until at least 40 days after cows have calved.
“Planning ahead will minimize the chance of making costly mistakes in estrous synchronization and AI programs. Protocols should be followed precisely,” Smith added.
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